I asked a friend of mine (a qualified mechanic) what he thought about your situation.
He replied...
"I'd be interested to see the truck on a data logger since I believe it's OBDII, it should have this capability. See if it's switching to closed loop when warmed up. How's gas mileage? I'd also shoot a laser temp sensor at all the cylinders (exhaust) and make sure their all around the same temp."
I hope this helps you.